Ortho Exam 3 Flashcards
Define the relationship b/w stress & strain
Internal state of the material
* calculated from force & deflection
Stress
- internal distribution of the load
- Force per unit area
Strain
- Internal distortion produced by the load
- deflection per unit length
Beams:
Arch wires or springs
1. Cantilever: supported at 1 end only
2. Support Beams: supported at both ends
If a Force is applied to a beam, how is the response measured?
Deflection (bending or twisting) produced by the force
Define the relationship b/w Force & Deflection
External Measurements
What are the 3 major properties of Beam/elastic materials?
- Strenggth
- Stiffness
- Range
Each can be defined by reference to a force-deflection or stress-strain diagram
Force-Deflection Diagram
- Stiffness: Slope of linear (elastic) portion
- Range: Distance along the x-axis to Yeild Point
- Yield Point: 0.1% of permanent deformation occured
- Springback: if the wire is deflected beyond the yield point; Does NOT return to original shape
- Failure Point: Wire breaks
Stress-Strain Diagram
Strength: the maximum load that the material can resist
* Represented by 3 different points: Proportional Limit, Yield Strength, Ultimate tensile Strength
Proportional LImit: most conservative measure
* The highest point where stress & strain still have a linear relationship (Hooke’s Law)
* Difficult to determine, yield strength is better indicator
Yield Strength: The intersection of
* stress-strain curve
* parallel line offset at 0.1% strain
Ultimate Tensile Strength: The max load a wire can sustain
* > yield strength
* Determines the max force the wire can deliver if used as a spring
Modulus of elasticity (E):
* slope of stress-strain diagram
* porportional to stiffness (E) and Springiness (1/E)
Modulus Of elasticity (E)
Slope of stress-strain diagram
* proportional to stiffness (E) and Springiness (1/E)
Elastic Limits of Materials
where permanent deformation is 1st observed
* b/w yield strength abnd proportional limit
Ultimate Tensile Strength
The max load a wire can sustain
* > yield strength
* Determines the max force the wire can deliver if used as a spring
Yield Strength
The intersection of
* stress-strain curve
* parallel line offset at 0.1% strain
Proportional limit
most conservative measure
* The highest point where stress & strain still have a linear relationship (Hooke’s Law)
* Difficult to determine, yield strength is better indicator
Define the relationship b/w strength, stiffness, and range
Strength= Stiffness x Range
Strength
The max load the material can resist
Measure in units of stress:
* SI Unit: Pascal (Pa) or MPa (Mega Pascal)
* gm/cm^2
* psi
Stiffness
Proportional to the slope of the linear portion (Elastic portion) of force-deflection curve
* The more vertical the slope: the Stiffer the wire
* The more horizontal the slope: the more flexible the wire
Range
The distance that we wire will bend elastically before permanent deformation occurs
* measured in mm
* If the wire is deflected beyond this point, does not go back to original shape
Springback
Determines Clinical Performance
* Measure along the x-axis
* if the wire is deflected beyond the yield point->Does NOT return to original shape
Resilience
Area under the stress-strain curve->out to the proportional limit
* Energy capacity of the wire: combo of strength & springiness
Formability
Amount of permanent deformation a wire can withstand before failing
* amount of permanent bending the wire will tolerate before it breaks
* distance along x-axis
Properties of an ideal wire for ortho:
- High strength
- Low stiffness
- High range
- High Formability
Reasonable Cost
Weldable or Solderable: - can attach hooks & stops to the wire
Precious Metal Alloys
Gold:
* to soft for all dental purposes
* advantage: easy to fabricate cast appliance
Alloys:
* could be used for ortho
* included: Platinum, Pallidium, Gold, Copper
Stainless Steel Wire
Replaced Precious metals bc:
* better strength & springiness
* equivalent corrosive resistance
Resistant to rust—due to
* High Chromium Content- 18% Chromium + 8% Nickel= 18-8 Stainless Steel
Steel Properties:
Controlled over a wide range by:
* Cold working-Hard Steel
* annealing-Softer Steel
Fully Annealed Stainless Steel wires (aka “Dead soft”) wire
* Soft
* Highly formidable
Steel archwires offered in a range of partially annealed states
* Yield Strength is progressive enhanced at the cost of formability
1.Super Grades of Steel wires
* most yield strength
* almost brittle- will break if bent to sharply
2.Regular grade of Steel wire
* can bend to any shape w/o breaking
Cobalt-Chromium Alloy
Aka Elgiloy
Advantage: can be supplied in a softer state
* thus more formable
Heat treatment:
* harden wires after being shaped;
* Increase Strength
After Heat treatment:
* the softest Elgiloy=regular stainless steel wire
* Harder initial grades=Super Stainless Steel
Rarely used today
